1. Technical Field
The present invention relates to a display device that controls gradation display by means of a sub-field driving scheme. In addition, the invention relates to a method for driving such a display device. Moreover, the invention further relates to an electronic apparatus that is provided with such a display device.
2. Related Art
In the technical field to which the invention pertains, a liquid crystal device is widely used as the display device of a variety of electronic apparatuses. A typical liquid crystal device of the related art displays an image as the result of a change in the optical transmission factor of the liquid crystal thereof. A few non-limiting examples of such a variety of electronic apparatuses having the liquid crystal device as its display unit include an information processing device, a television, and a mobile phone. A liquid crystal device of the related art is, in the typical configuration thereof, provided with a plurality of pixel electrodes each of which is provided at the intersection formed by a scanning line, which extends in a row direction, and a data line, which extends in a column direction. A pixel-switching element such as a thin film transistor (TFT) or the like is provided at a position corresponding to each of the intersections formed by a plurality of scanning lines and a plurality of data lines. Specifically, a TFT pixel-switching element is interposed between the pixel electrode and the data line in such a manner that it corresponds to each of these intersections. On the basis of a scanning signal that is supplied via the corresponding scanning line, the TFT pixel-switching element switches over the ON/OFF state of a connection therebetween. A counter electrode is provided opposite the pixel electrode with the liquid crystal being sandwiched between the counter electrode and the pixel electrode. When a voltage is applied between the pixel electrode and the counter electrode in accordance with the gradation of an image signal, the orientation state, that is, the alignment state, of the liquid crystal changes in accordance with the level of the voltage applied thereto. As a result thereof, the amount of light that passes through the liquid crystal at the corresponding pixel changes so as to enable a desired gradation display.
In the typical configuration of a liquid crystal device of old conventional art, an image signal that is applied to the data line has a voltage format that corresponds to gradation, that is, the signal format of an analog signal. For this reason, a liquid crystal device of such old conventional art requires a D/A conversion circuit, an operational amplifier, and the like, as the peripheral circuit components thereof. Such a configuration is disadvantageous not only in that the production cost thereof is relatively high but also in that it is practically impossible, or at best difficult, to display an image in a uniform manner. As an effort to provide a technical solution to such a conventional problem, these days, a sub-field driving scheme is proposed. The sub-field driving scheme adopts a digital format for the driving of liquid crystal. Specifically, in a typical sub-field driving scheme, each one field is divided into a plurality of sub fields on a time axis. In each of the plurality of sub fields, either an ON signal or an OFF signal is applied depending on the gradation of each of pixels. Such a sub-field driving scheme is described in, for example, JP-A-2003-114661.
Specifically, JP-A-2003-114661 discloses a technique of displaying an image with gradation that is finer than the number of sub fields that are contained in one field, which is achieved by fine-controlling the change in the optical transmission factor of the liquid crystal in each sub field. Assuming that the number of sub fields that make up one field is denoted as “n”, the number of gradations obtained under a usual pulse-width control is limited to (n+1). Under the usual pulse-width control, each field has a pattern in which ON sub fields follow one after another. In contrast, according to the above-identified art that is described in JP-A-2003-114661, since OFF sub fields are mixed therein, it is possible to represent/display gradations the number of which is considerably larger than (n+1), which is available under the usual pulse-width control.
As a reference for converting the gradation of display-target image data into sub-field data, a lookup table is used. A lookup table is a table that shows the gradation of a display-target image (e.g., gradation represented in eight bits) and the ON/OFF pattern of sub fields (e.g., thirty-two sub fields that make up one field) in association with each other. In such a lookup table, as the sub-field ON/OFF pattern thereof, a pattern that is suitable for representing gradation has been obtained in advance theoretically and empirically, that is, by way of an experiment, in accordance with the characteristics of liquid crystal or the like.
In connection therewith, generally speaking, the characteristics of liquid crystal changes depending on temperature. FIG. 12 is a graph that illustrates an example of the characteristics of liquid crystal, specifically, the relation between gradation and optical transmission factor thereof under a plurality of temperature conditions. In the illustrated example, the temperature of a liquid crystal panel is assumed to be 40° C., 50° C., and 60° C. As understood from characteristic curves shown therein, the characteristics of a liquid crystal under one temperature condition differ from that of another. For this reason, image data having the same single gradation could be displayed with brightness levels that are different from one another depending on temperature conditions.
In order to correct variations/differences in the optical-transmission-factor characteristics of liquid crystal that are attributable to variations/differences in temperature conditions so as to obtain a uniform display, it is necessary to preset, and store in a memory, a set of a plurality of lookup tables so as to correspond to a plurality of actual temperature conditions. This inevitably requires a large memory capacity.